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NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

Damage characterization of asphalt in laboratory by ultrasonic wave propagation

Adrien HOUEL1, Laurent ARNAUD2

1 CETE de Lyon, LRPC de Lyon, 25 avenue François Mitterrand, Case n°1, Bron cedex, 69674, France, [email protected] 2 Université de Lyon, Lyon, 69003 France ; Ecole Nationale des Travaux Publics de l’Etat, CNRS URA 1652, Département Génie Civil et Bâtiment, rue Maurice Audin, Vaulx-en-Velin cedex, 69518, France

Abstract A non-destructive measurement technique is used to follow the mechanical evolution of

asphalt pavement throughout mechanical tests. The test is based on the propagation of ultrasonic waves (compressional and shear waves). With an inverse analysis, it is then possible to calculate and follow up the evolution of mechanical parameters of the materials such as viscoelastic modulus G, œdometric modulus… Indeed, this technique makes it possible to obtain mechanical characterization of bituminous concrete throughout complex modulus tests. The paper draws a parallel between results of complex modulus from strain measurements (extensometers or strain gages) and from this method. Moreover, ultrasonic wave propagation makes possible to follow the intern temperature of the material and to determine its homogeneity inside. Finally, this technique enables to plot at each instant the rheological curve of the material (G’, G’’) and to follow up the time dependent evolution of damage throughout fatigue tests. This non-destructive test is applied on two different laboratory devices: the five-point bending test and cylindrical cores. The presented results provide excellent perspectives for using this complementary method on different bituminous concrete tests.

Résumé Une méthode non-destructive est utilisée afin de suivre le comportement mécanique des

couches d’enrobé bitumineux, méthode couplée à des essais classiques de laboratoire sur ce type de matériaux. Ces essais sont issus de la propagation d’ondes ultrasonores (ondes de compression et de cisaillement). Par analyse inverse, il est possible de calculer et suivre l’évolution de caractéristiques mécaniques comme le module complexe, le module œdométrique, … Cette méthode peut être en effet utilisée pour caractériser les enrobés bitumineux pendant les essais de module complexe. Ce papier compare les résultats de module complexe, à partir des mesures des déformations et à partir des essais non-destructifs. De plus, les ondes ultrasonores permettent de suivre la température interne du matériau et de déterminer son homogénéité à cœur. Enfin, cette technique permet de suivre à chaque instant l’endommagement du matériau durant un essai de fatigue en mesurant et calculant l’évolution des modules mécaniques. Ces essais non-destructifs sont appliqués à deux dispositifs expérimentaux de laboratoire : l’essai de flexion 5 points et les essais classiques sur éprouvettes cylindriques d’enrobés bitumineux. Les résultats présentés fournissent d’excellentes perspectives dans l’utilisation de cette technique complémentaire aux essais sur enrobés bitumineux.

Keywords Cracks, five-point bending test, viscoelasticity, steel orthotropic plate.


1. Introduction Mechanical properties of a material can be sensed by ultrasonic waves. Two kinds of

ultrasonic waves exist inside a homogeneous and isotropic solid: P-waves or compression waves, and S-waves or shear waves through bituminous mix are studied in this paper. This interesting technique is non-destructive. It makes it possible to evaluate the mechanical evolution of material parameters with time dependant properties, particularly throughout fatigue tests. The elastic theory allows calculation of the P-wave and S-wave velocities in a homogeneous and isotropic solid that can also express as function of the complex modulus M and Poisson's ratio [1].

Nevertheless, bituminous mix cannot be considered as an elastic medium because it is a viscoelastic material. Thus, attenuation parameters such as peak amplitude can be analyzed and offer useful information about the material’s characteristics. As a result, ultrasonic wave propagation is an efficient tool for mechanical investigation.

The main difficulties to analyze ultrasonic wave propagation in heterogeneous materials like bituminous concrete consist in:

• determining the choice of sensors and their excitation frequency due to the number of multiple scattering processes and the frequency-dependant attenuation and dispersion of elastic waves [2],

• obtaining the best sensor location to evaluate the best area where asphalt is likely to crack.

This paper shows that it is possible to follow up the evolution of mechanical parameters of bituminous concrete thanks to ultrasonics throughout two experimental devices:

• the five-point bending fatigue test, and • the fatigue test on bituminous concrete cylindrical cores.

2. Experimental method

2.1. Ultrasonic tools P-waves and S-waves are used on the two experimental devices presented in the followed

part. In practice, we place an emitter and a receiver transducers on two accessible sides. In each case, several parameters are measured from ultrasonic signals (Fig.1) [3]:

• The arrival time (∆t). We can make the difference between the propagation velocity of P-waves Vp (Vp=∆t/d) and wave velocity cp (cp=∆t/dmin), since Vp only equals cp when the wave travels along the shortest distance dmin between both transducers. The same reasoning holds for S-waves. We assume that the path length of the wave is a straight line going from the center of the emitter to the center of the receiver. So the propagation velocity V equals the wave velocity c.

• The maximum amplitude of ultrasonic compression and shear waves recorded. • The Fast Fourier Transform (FFT) of the received signal.

Thus, by inverse analysis, we have under the assumption of elasticity in an infinite continuum:

and G (1) M = ρVp2 = ρVs

2

where M is the constraint modulus, G is the shear modulus and ρ is the mass density of the medium.

Moreover, as bituminous mix is a viscoelastic material, the calculation of the modulus can be improved with the assumption of linear viscoelasticity.


Figure 1. Received signal of P-wave propagation: time delay and damping [3]

Different issues have been overcome. Firstly, the wavelength must be not only smaller than the size of the sample, but also larger than the granular size. A compromise should be done to get good results. P-wave velocity through the asphalt is about 4000 m/s at -10°C and 3600 m/s at +10°C and the velocity of the S-wave is around 2100 m/s at -10°C and 1800 m/s at +10°C. Given that the wavelength is the ratio between velocity and frequency, we can choose the appropriate frequency excitation for each transducer. Wavelengths values are given in Table 1. The wavelength of S-waves is of the order of the specimen size but received signals could be analyzed. The best compromise in excitation frequencies was found.

Table 1. Frequency excitations and wavelengths of transducers used throughout the fatigue tests on bituminous mixes

Temperature Wavelength λ -10°C +10°C

P-wave (50 kHz) 7 cm 6 cm S-wave (10 kHz) 15 cm 20 cm

Secondly, throughout a fatigue test, the material is overheated. Therefore, since

bituminous concrete is a viscous medium, we have to account for the temperature variation and separate this phenomenon from fatigue damage. Results show that there is a linear relation between the material temperature and the velocity of P-wave [3]. For instance, an increase in temperature of +1°C corresponds to a decrease in modulus of 150 MPa, which represents 5 to 10% with regard to damage from the fatigue effect. The temperature effects cannot be forgotten, but we neglect these consequences on wave propagation throughout the fatigue tests, insofar as the fatigue effect plays a very prominent part.

2.2. The experimental devices We used two different fatigue tests: the first one is the five-point bending test that is an

inhomogenous test, whereas the second one on cylindrical cores is homogenous.

2.2.1. The five point bending test (FPBT) [4-7] The FPBT consists in testing a specimen reproducing the area located on either side of an

orthotropic plate stiffener. It consists in a fatigue under negative bending test over several million cycles. A 60 mm-thick asphalt mix is laid on a steel plate (580 x 200 mm) constituted by a 12 or 14 mm-thick steel plate reinforced at the center with a welded stiffener. A sealing sheet between the steel plate and the asphalt pavement forms a 3 mm-thick layer. The steel plate rests on three supports, two are simple supports and the third one in the center is an embedding (Fig.2left).

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009 As the sample is both in traction at the center (zone 1) and in compression under shoes

(zone 2), it is not possible to study the wave propagation along a longitudinal path. Thus, the transducers are not placed longitudinally, but in the central area on the lateral faces, in order to have a continuous monitoring of the asphalt pavement modulus where cracks are likely to appear (Fig.2right). Moreover, two LVDT (Linear Variation of Differential Tension) sensors are positioned on the upper face at the center of the specimen. By using LVDT’s with different ranges, both accuracy and a large detection area are ensured. A first LVDT is certain to have a measurement zone where cracks are likely to appear, and a second senses the displacements more precisely (LVDT n°1: ±2.5 mm with ∆l=60 mm and LVDT n°2: ±1.0 mm with ∆l=30 mm).

Figure 2. Set up for the FPBT (cross section) and positions of both LVDT on the upper side of the asphalt wearing and wave transducers on lateral faces in the tensile zone [4,8]

2.2.2. Fatigue tests on cylindrical specimens A homogeneous fatigue test has been developed on bituminous concrete cylindrical cores

by means of a new set up. It makes it possible both to propagate ultrasonic waves and to measure strains at the same time throughout the test. Each sample is stuck by its ends on a metallic support on which a P-wave transducer is positioned in contact with the asphalt (Fig.3). Strains are measured at the center of the sample by means of three LVDT’s (±1.0 mm ∆l=40 mm) positioned at 120° around the sample.

Figure 3. Experimental set up for the fatigue test on cylindrical sample [5]


3. Results Results of energy loss from P-wave propagation show that non-destructive tests are as

efficient as measurements of the displacement sensors (Fig.4). It points out a classic fatigue curve divided into three zones that can be observed in each case, for either velocity of P-waves, or the received energy, or the calculated modulus:

− firstly, the material is submitted to the loss of its properties due to overheat and damage at the same time,

− secondly, signals stabilize that corresponds to the creation of microcracks in the bituminous concrete,

− thirdly, a macrocrack is induced and the bituminous concrete suddenly goes to the breaking.

In addition, the test was stopped, and the shape exhibits a classic result for asphalt concrete: if the material is not loaded, it self-repairs, i.e. it recovers a part of its mechanical properties. The energy recovered can also be due to the decrease in temperature inside the bituminous concrete.

Figure 4. Strain average and energy loss from waves propagation throughout the fatigue

test on cylindrical specimen at -10°C [3]

Figure 5. Norm of the complex modulus calculated from P-wave velocities and their damping by inverse analysis throughout the FPBT on two bituminous mixes at +10°C [5]

An example of results of wave propagation throughout the FPBT is presented in Fig.5. It shows the mechanical evolution of the bituminous concrete through the results of the evolving modulus from P-wave and S-wave transducers. P-wave and S-wave velocities and


their amplitude decrease clearly as the number of load cycles increases. Then signals stabilize before significant amplitudes decrease.

For the Millau viaduct bituminous concrete mixture, the calculated modulus is about 28,500 MPa at the beginning, and decreases to 27,400 MPa after 800,000 cycles. Then, it stabilizes until 3 million cycles before significant decreases. This last stage reveals the appearance of cracks that are created and propagated as far as the middle of the thickness of the bituminous concrete pavement. In the second case for a classic road bituminous concrete, the decrease appears throughout the two hundred thousand first cycles decreasing from 30,000 MPa to 26,500 MPa.

Therefore, wave propagation is an efficient way to compare and to monitor the evolutive behavior of different wearing courses at each instant of the FPBT, and helps damage curve plotting.

4. Conclusions In this paper, ultrasonics technique are presented and tested on bituminous concrete, a

heterogeneous and viscoelastic material. It is shown that: • ultrasonic wave propagation can be used to monitor the evolutionary intrinsic

mechanical characteristics of the material, • from the measurements of wave velocities and damping coefficients, the complex

modulus is calculated at each instant during the test and the damage evolution curve can be plotted as a function of the material.

• non-destructive tests by wave propagation on cylindrical specimens are as efficient as measurements with displacement sensors, and results are consistent with those realized throughout the five-point bending test.

References 1. Santamarina, J. (2001) "Soils and Waves", John Wiley and Sons, pp.185-193, 291-292. 2. Hauwaert, A. V., Thimus J.-F., Delannay F. (1998) "Use of ultrasonics to follow crack

growth", Ultrasonics, Vol. 36, pp.209-217. 3. Houel, A., Arnaud, L. (2007) "Damage characterization of asphalt concrete specimens by

ultrasonic P and S-wave propagation in laboratory", Proc. of the Int. Conf. on Advanced Characterisation of Pavement and Soils Engineering Materials, Ed by A. Loizos, T. Scarpas and I. Al-Qadi, 20-22 June 2007, Athens, Greece, pp.185-193.

4. Arnaud, L., Houel, A. (2007). "Fatigue damage of asphalt pavement on an orthotropic bridge deck: mechanical monitoring with ultrasonic wave propagation", Int. J. of Road Materials and Pavement Design, Vol. 8-3, pp.505-522.

5. Houel, A. (2008) "Endommagement à la fatigue et fissuration mécanique des enrobés bitumineux sur dalle orthotrope" (in french), PhD Thesis, Ecole doctorale MEGA-ENTPE, Lyon, France, June 2008, 252p.

6. Houel, A., Arnaud, L. (2008) "The five-point bending test: a way to the dimensioning of the asphalt layer on steel orthotropic decks", Proc. of the Int. Orthotropic Bridge Conference, 25-29 August 2008, Sacramento, United States.

7. Héritier, B., Olard, F., Saubot, M., Krafft, S. (2005) "Design of a specific bituminous surfacing for orthotropic steel bridge decks: application to the Millau viaduct", 7th Symposium on Bearing Capacity of Roads, Railways and Airfields, Trondheim, Norway.

8. Laajili H. (2003) "Caractérisation des enrobés bitumineux comme couche de roulement sur tabliers d’ouvrage d’art métallique" (in french), Master in Civil Engineering, Ecole doctorale MEGA-ENTPE, Lyon, France.

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